Silane grafting/water crosslinking can provide crosslinked polyolefin materials at lower cost than chemical crosslinking using an organic peroxide and hence has been widely used for crosslinking of wire and cable materials.
According to this method, a polymer is graft copolymerized with a vinylalkoxysilane in the presence of a minor amount of an organic peroxide as a graft reaction initiator in a molding machine (for example, an extruder) at a high temperature, and the wire and cable is then exposed to a high temperature and high humidity atmosphere (or hot water) to induce hydrolysis and condensation of the alkoxysilane grafted onto the polymer through the action of a silanol condensation catalyst, such as dibutyltin dilaurate, which has been mixed into or has permeated from the surface of the polyolefin molding into the interior of the polyolefin molding, thus causing crosslinking.
The inner and outer semiconductive layers of plastic-insulated power cables, particularly high voltage crosslinked polyethylene insulated power cables, are generally formed by extrusion. These semiconductive layers should be in intimate contact with a crosslinked polyethylene insulator from the viewpoint of avoiding corona discharge. In treating the terminal of the cable, however, the outer semiconductive layer should be easily stripped without damage to the insulator. For this reason, what is required of the outer semiconductive layer is that the layer is usually in intimate contact with the insulator and if necessary can be easily separated. A composition composed mainly of an ethylene/vinyl acetate copolymer has hitherto been extensively used as a strippable semiconductive resin composition for the outer semiconductive layer.
Use of the outer semiconductive layer on the insulator crosslinked by silane grafting/water crosslinking enhances the adhesion between the insulator and the outer semiconductive layer through the action of a functional group, such as silanol, making it difficult to strip the outer semiconductive layer. When the content of vinyl acetate in the ethylene/vinyl acetate copolymer is increased (for example, to not less than 45% by weight) to facilitate the stripping, acetic acid is likely to be eliminated upon heating at the time of molding. This poses a problem of deteriorated heat resistance and, in addition, discoloration of copper tape as a shielding layer and copper wire as a conductor by the eliminated acetic acid.